An energy storage system, such as a flow battery, may include one or more cells that operate to store energy provided from a source, and to discharge energy to a device to do work. Each cell may have a cathode, an anode and a separator disposed between the cathode and anode for separating chambers of the cell that receive electrolytes. The separator may permit ionic flow between the cathode and anode to facilitate energy storage in the system, as well as discharge of energy from the system.